1. Field of the Invention
The present invention relates to methods for accurate and reliable detection of leaks in pressurized pipe systems containing a liquid such as water, petroleum fuels and products, and other hazardous and non-hazardous substances, and more particularly to a method achieving high performance due to accurate compensation of product temperature changes that occur during a test.
2. Brief Discussion of Prior Art
There are a number of different types of pressure-based methods that are used to detect leaks in underground pressurized pipelines containing petroleum fuel or any type of liquid. A loss of liquid from the line due to a leak will produce a drop in pressure. A leak is declared if the pressure drop exceeds a predetermined threshold value. The magnitude of the pressure drop due to a leak is a function of and proportional to the volume of the liquid in the line, the bulk modulus of the pipeline system, and the initial pressure of the line. Thus, the pressure drop is larger in a smaller line than in a larger line for the same size leak. This is important because the pressure changes due to product temperature changes are independent of line volume.
Conventional Pressure Test
The most common type of pressure test is a pressure-decay or hydrostatic test. The method is to isolate the line to be tested from tanks or other line segments connected to it by valves or valve blinds, fill the line with a liquid or use the existing liquid in the line, pressurize the line, and then monitor the pressure changes over time. This pressure testing method is used for evaluating the structural integrity of a pipe. The main applications of the method are in transportation and transmission pipelines. Because of the inherent errors in this type of test, such a test was not originally intended to be used for leak detection.
Erroneous results occur in a pressure test if (a) any vapor is trapped in the line (or appurtenances attached to the line) or (b) the temperature of the fuel changes during a test. While both effects are acknowledged in the test procedure, no methods are offered to compensate for their effects. Also, the method described in these standards does not indicate what threshold to use to declare that the line is leaking, i.e., how large a pressure drop is required before the line is suspected of leaking. Over the years this method has been frequently applied to a wide range of pipelines, both small and large, but without much success for detecting small leaks.
Ambient Product Temperature Changes
One reason that this approach has not been successful is that a drop or rise in pressure can also occur if the temperature of the fuel (or liquid in the line) is also changing. An increase in temperature will cause the pressure in the line to increase. If these thermally induced pressure changes are large, they can mask the presence of a leak and result in a missed detection. A decrease in temperature will cause the pressure in the line to decrease. If these thermally induced pressure changes are large, they can falsely indicate the presence of a leak and result in a false alarm.
Underground petroleum fuel lines can experience large, nonlinear temperature changes, which produce large, thermally induced pressure changes, because the coefficient of thermal expansion for petroleum fuels is large and the temperature of the product brought into the line can be very different than the temperature of the fuel in the line or the ground surrounding the line. A new temperature condition is generated any time fuel from a storage tank is transferred through a pipe.
FIG. 1(a) is a time series showing the typical thermal behavior of product brought into a line at a warmer temperature than the backfill and soil surrounding the line; FIG. 1(b) shows the time series of the rate of change of temperature. The thermally induced pressure changes are proportional to temperature changes, and scale according to the bulk modulus (compressibility) of the line and the coefficient of thermal of expansion of the liquid in the line. The volume of product in the line affects the rate of change of the temperature in the line. Thus, the two time series in FIG. 1 also illustrate the thermally induced pressure changes that occur in the line. In the present disclosure, this type of product temperature and line pressure change will be referred to as an ambient thermal change to distinguish it from product temperature and line pressure changes produced by changing the pressure in the line.
The observed curvature in both the temperature (and the pressure) and the rate of change of temperature (or the rate of change of pressure) curves in FIG. 1 clearly illustrate the nonlinear changes in product temperature that occur during a test. When high performance is desired, testing with conventional pressure-decay methods, which do not compensate directly for the product temperature changes, cannot be initiated until the rate of change of temperature is sufficiently small that the thermally induced pressure changes are negligible. This means that the line must be taken out of service for whatever length of time is necessary to reach this stage of negligible thermal changes. Small pipelines at retail service stations may require a waiting period of 2 to 12 h. The larger lines at bulk fuel storage facilities may require a waiting period of 12 to 36 h, and the larger lines found in airport hydrant systems may require a waiting period of many days or longer.
This approach for minimizing the impact of fuel temperature changes with a waiting period has adverse operational and performance implications. First, transfer operations may need to cease for an unacceptably long period of time. Second, there is no way to guarantee that a presumably adequate waiting period is in fact sufficiently long for thermal changes to dissipate. Third, even if the waiting period is adequate, there is no way to verify quantitatively that the rate of thermal change is negligible or to verify that product temperature has not changed in response to other heat sources and sinks (e.g., heating or cooling of a section of an underground pipe that is exposed to sun or clouds).
The use of a waiting period to minimize the thermally induced errors in a pressure test is only practical for use on pipelines with small diameters or small capacities, such as those found at petroleum fuel service stations. Even for these pipelines, this approach has had only limited success. The use of a waiting period is also not useful if quick tests are to be conducted. The use of a waiting period is not practical for large diameter or large capacity lines found at bulk fueling facilities, in airport hydrant systems, or in transportation or transmission pipelines, because it could take many days or longer for the thermal changes to become negligible.
For these larger pipelines, accurate leak-detection tests can only be performed if the thermally induced pressure changes are compensated for. One compensation approach is to measure the temperature changes of the fuel in the line, estimate by standard hydraulic computations the magnitude of the pressure changes produced by these temperature changes, and then subtract these thermally induced pressure changes from the measured pressure changes. If the temperature compensation is accurate, then only the leak-induced pressure changes remain.
This temperature-compensation approach to performing a pressure test has a number of serious technical and implementation problems. First, it is very difficult to obtain accurate measurements of the fuel temperature changes along the length of the pipe. These measurements are necessary to account for any differences in the temperature conditions along the length of the line. Typically, no more than one temperature sensor is used, even though the pipe may be many miles in length and may be affected by many different thermal environments. Second, the bulk modulus and volume of the product in the line must be accurately known.
Trapped Vapor
The presence of trapped vapor or any appurtenance (like a surge suppressor) that may change the compressibility of the line as the pressure in the line changes can lead to a test result that is impossible to interpret. As the volume of trapped vapor in a line increases, the magnitude of the pressure change that occurs due to a given leak or product temperature change decreases. The pressure changes from large leaks can be reduced to undetectable levels because of small amounts of vapor in the line. The presence of vapor, which is difficult to quantify in terms of its volume, makes the results of pressure tests totally ambiguous, because the pressure drop can range from a very small value to a very large value for the same size leak and same initial test pressure.
FIGS. 2 and 3 show the difference in the pressure drop with and without trapped vapor in a 3,133-gal line and 12,500-gal line produced by a very modest temperature change over a 1-hour period and a 4-hour period. These differences occur even if the thermally induced pressure changes due to the ambient temperature changes are compensated for. If trapped vapor or surge suppressors are present in the line and the volume of the trapped vapor is not known, then pressure tests become highly inaccurate and should not be used.
Pressure induced temperature changes also seriously degrade the performance of a pressure test is produced any time the pressure in the line is changed. A xe2x80x9csmallxe2x80x9d temperature change is associated with any pressure change. This temperature change produces a perturbation in and affects the rate of change of the underlying ambient product temperature field.
FIG. 4(a) illustrates pressure change as a function of time, and FIG. 4(b) shows a corresponding (exaggerated) pressure-induced thermal perturbation resulting from increasing and decreasing the pressure in the line. The underlying ambient product temperature as it would have been had there been no pressure change is shown by the dashed line. These pressure-induced product temperature changes, which may be several hundredths to several tenths of a degree Centigrade, occur because the pressure change compresses the liquid or causes it to expand. This effect is even larger if there is trapped vapor in the line, or if the line and appurtenance on the line are more compressible than the liquid in the line. Once a temperature perturbation is induced, the change in temperature over time is controlled by the difference in temperature between the fuel in the line and the surroundings. This means that the pressure changes that occur are not independent but are coupled with the ambient changes.
These thermal perturbations in temperature may take tens of minutes or longer to come into equilibrium with the underlying ambient product temperature field. The magnitude of the temperature perturbation at a given point in time is dependent on the magnitude of the pressure change, the time that elapses between the pressure change and the measurement period, the volume of product in the pipe system, and the system characteristics that control the rate of change of temperature of the product in the pipe (e.g., pipe diameter and pipe wall material, type of product in the pipe, and the type, characteristics, and condition of the backfill and soil surrounding the pipe). In many instances, after tens of minutes, the rate of change of temperature caused by these anomalous phenomena is too small to measure with most common temperature measurement sensing systems.
Two-Pressure Test
In U.S. Pat. No. 4,608,857, Mertens describes a pressure test method for compensating for fuel temperature changes during a pressure test without directly measuring the temperature changes in the line. This test is conducted using a test comprised of three measurement segments and two different pressure levels. As shown in FIG. 5(a), the initial pressure levels of the first and third measurement segments are the same (denoted by the dot) and the initial pressure level of the second measurement segment is different (denoted by the dot). The pressure changes (or rate of change of pressure) from the first and third measurement periods are then averaged and subtracted from the pressure changes (rate of change of pressure) measured during the second measurement period to obtain a temperature-compensated pressure change (rate of change of pressure). This compensated pressure change difference is then compared to a threshold to determine whether or not a leak is present; the threshold is referred to in Mertens as the xe2x80x9ctime standard allowed pressure change difference value.xe2x80x9d Mertens states that xe2x80x9cthe influences of changes in temperature on the pressure curve is almost completely eliminated.xe2x80x9d This assertion is only true if the second derivative of the unperturbed pressure field is zero or very nearly zero. Using the analytical assumption that the second derivative of the pressure field is zero is a useful approximation only over a very short time period (less than 20 minutes). Tests that use data taken over a longer period of time must take into account the non-zero nature of the second derivative.
Mertens describes a two pressure, two measurement segment pressure test method for compensating for fuel temperature changes, but this method requires the unrealistic assumption that the pressure changes are linear during a test. If the fuel temperature changes can be assumed to be linear, then the thermally induced pressure changes can be compensated by subtracting the rate of change of pressure during the first measurement segment with the rate of change of pressure during the second measurement segment. Since the fuel temperature changes are not linear, this approach is unreliable and will not work if small leaks are to be detected.
The pressure-induced thermal perturbations produce a systematic error, or bias, in the temperature-compensated rate of change of pressure computed from the pressure data using either of these two methods described by Mertens. For a given liquid product, the magnitude of this systematic error depends on the difference between the low and high pressures used to conduct a test, the time that elapses between any pressure changes and the subsequent measurement periods, and the volume of product in the line. If the liquid product in the line changes, then the magnitude of the systematic error also depends on the magnitude of the coefficient of thermal expansion of the liquid and the bulk modulus of the liquid. Whether or not this systematic error can be tolerated during a test depends on the performance desired of the system (i.e., the smallest leak to be detected). If it cannot be tolerated, then an estimate of this systematic bias needs to be minimized or measured, and removed.
Mertens describes this effect as creep and claims that this pressure change has a time constant of about 0.5 to 1 h and states that if a quick test is to be conducted, then this pressure change due to creeping must be compensated for, because it can produce a pressure change identical to a leak.
This systematic error or creep is compensated for by Mertens using a calibration procedure. In a short test, the magnitude of this thermal effect may be 2 to 10 times larger than the magnitude of the leak to be detected. In order to compensate for an effect of this magnitude, the calibration must be conducted with great accuracy. Mertens describes a compensation process in which an xe2x80x9cempirical constantxe2x80x9d that is dependent upon the difference between the initial pressures of the two test pressures is determined from measurements on the line to be tested, when it is known to be leak free and then used to adjust (reduce) the measured pressure difference obtained during a test. As a consequence, the method taught by Mertens is mainly useful for implementation on new pipelines where the integrity of the line is known initially or on an existing line previously tested by another leak detection method. This limits the application of this methodology for leak detection, because the integrity of most of the lines that need to be tested for leaks, by definition, is of course unknown.
Mertens recognizes that this creeping pressure change is a function of the bulk modulus (i.e., compressibility) of the line, the magnitude of the temperature change, and the coefficient of thermal expansion of the product in the line. However, he did not recognize that this pressure change will impose a nonlinear, time-dependent change of pressure and that the magnitude of the change is also a function of the difference in temperature between the fuel and the surroundings (e.g., ground) and the spatial temperature distribution in the surrounding environment and the line. The rate of change of temperature and therefore, the rate of change of pressure can be different, even for the same initial temperature difference, depending on the spatial distribution of temperature in the ground. The temperature conditions in the ground are highly dependent on the transfer history of the fuel.
The systematic error or creep can be minimized by a variety of other approaches besides calibration on the line to be tested. The magnitude of the systematic error can be reduced by reducing the magnitude of the pressure difference used in testing the line or by increasing the time between any pressure change and the subsequent measurement period. Each of these approaches has a number of drawbacks that can impact the performance of the method or its application to the particular line to be tested.
Since the method described measures the difference in the rate of leakage at two different pressures, reducing the magnitude of the pressure change reduces the magnitude of the signal that is to be detected in addition to reducing the noise (or xe2x80x98creepxe2x80x99). Since the magnitude of the signal decreases faster than the magnitude of the noise, the performance of the method is degraded under these circumstances. Also, reducing the magnitude of the pressure change is not always possible if the line must be tested at a prescribed pressure or if the pressure difference is not sufficient to detect the leak rate of interest.
Increasing the interval between any pressure change and the subsequent measurement period is an effective means of addressing the adverse effects of the perturbation, because these temperature changes decrease with time. However, if the duration of the test becomes too long, the accuracy with which the methods described above compensate for the ambient thermally induced volume changes is degraded. This is because the rate of change of temperature does not decrease linearly over long periods of time. Thus, for optimal performance, a balance must be found between the length of the intervals (between the measurement periods and the pressure changes) and the total length of the test.
The maximum-size line that can be tested with the methods taught by Mertens will depend on the performance requirements, the pressure difference, and the time between the pressure changes and the measurement periods and line volume.
For accurate leak detection tests, accurate compensation of the thermally induced pressure changes that occur during a test is required (1) because the fuel in the line is different than the surrounding environment and (2) because of creeping in the line produced by changing the pressure during a test. The methods used by Mertens to compensate for both of these effects greatly limit the application of the method. First, the higher-order nonlinear thermally induced pressure changes, whether they are caused by differences in temperature between the fuel and the environment (ground) or by creeping, can produce large enough errors to prevent this method from finding small leaks. Mertens methodology does not recognize that creeping is also a source of nonlinear temperature changes. Second, the so-called creeping that occurs in a line over a time interval is a very large and important source of error in the method described by Mertens, because of the large time required to perform the Mertens"" method due to the need to change the pressure three times to complete a test.
It is therefore an object of the present invention to provide a method for reliable and accurate detection of leaks in pressurized pipe systems containing liquids, including water, petroleum products, and hazardous and nonhazardous substances.
It is another object of the present invention to provide a method of compensating for the thermal expansion and contraction of a product in a pipe and of the pipe itself.
A further object of the present invention is to provide a method of estimating the error in compensating for the thermal expansion and contraction of the product in the pipe and of the pipe itself.
A still further object of the present invention is to provide a method for detection of leaks in a pressurized pipe system containing liquids that is designed to work well when the rate of change of product temperature is nonlinear.
Yet another object of the present invention is to provide a method for the detection of leaks in pressurized pipelines containing liquids that is designed to minimize the effects of pressure-induced thermal perturbations to the ambient product temperature field.
A further object of the present invention is to provide a method for testing a pressurized pipe system for leaks by collecting and analyzing data at a minimum of two pressures and with as few as two measurement periods.
Another object of the present invention is to provide a method that can be used to test pipe systems for leaks without the use of any a priori calibration or empirical data on analytical or empirical models for, or general knowledge about the status of the pipe system to be tested or other similar pipe systems when in a nonleaking condition.
Briefly, a preferred embodiment of the present invention includes a method of detecting a leak in a pipeline systems, wherein a measurement is preferred to determine the difference in the rate of change of pressure due to a leak between one pressure level and at least one other pressure level, after compensation has been made for thermally induced changes in the pressure in a pressurized pipeline system, including the steps of pressurizing the pipeline system to a first pressure level, and measuring the changes in pressure in the pipeline system that occur over a first measurement period, and pressurizing the pipeline system to at least a second pressure level, and measuring the changes in pressure in the pipeline system that occur over at least a second measurement period. A computation is then performed of the difference in the temperature compensated rate of change of pressure between one pressure level and at least one other pressure level from the measured pressure data at the pressure levels, including a correction for the thermally induced non-linear changes of pressure between the measurement periods, wherein the difference in the temperature compensated rate of change of pressure between the pressure levels is computed from the rate of change of pressure measured during the measurement periods (first derivative of the pressure data or rate of change of pressure) and higher order derivatives of the pressure data.